A Light-Emitting Diode (LED), as used herein, is a semiconductor light source for generating a light at a specified wavelength or a range of wavelengths. LEDs are traditionally used for indicator lamps, and are increasingly used for displays. An LED emits light when a voltage is applied across a p-n junction formed by oppositely doped (and typically compound) semiconductor layers. Different wavelengths of light can be generated using different materials by varying the bandgaps of the semiconductor layers and by fabricating an active layer within the p-n junction.
LEDs are semiconductor based. For a given drive current, light output varies from chip to chip, and also varies over the life of each chip. Light output also varies inversely with temperature, but not uniformly for each color. Finally, in a block of LEDs of a given color, the light output will vary if one or more of the LEDs fails. Given all the factors which can affect the color balance of any array of LEDs, it would be desirable to automatically monitor and regulate the color balance, especially in a white-light emitting module.
In an array of LEDs of the same color, for example a traffic light, variations in light output based on temperature can be compensated by varying the current supplied to the array. This scheme would be cumbersome in a module having LEDs in a plurality of colors, because the variation in light output due to temperature is not the same for the various colors.
It would be desirable to be able to compensate for the different temperature variations in output in a multicolor array of LEDs. For example, it would be desirable to automatically control the chromaticity of a white light emitting module comprising red, blue and green LEDs, without regard to the factors which cause the light outputs of the individual colors to vary.
It would further be desirable to automatically control the variation of each color LED without resorting to a spectrally resolving light measuring system, such as a photodiode and filter, for each of the respective colors.